Our objective is to investigate excitatory pathways of the brain which are inhibited by 2-amino-4-phosphonobutanoic acid (APB). This compound has close structural analogies with glutamic acid. Glutamic acid or related compounds are suspected to be neurotransmitters for major excitatory pathways in the brain. The compound APB is a highly potent and selective inhibitor for some of these pathways. A specific aim is to chemically synthesize analogues, designed to elucidate structure-function relationships for APB neural receptors and binding sites. Proposed compounds will include those designed to define spatial requirements of functional moieties of APB and glutamate for interaction with receptors and binding sites. They will also include conformationally-restricted APB and glutamate analogues and analogues with alkylating- or photoaffinity-labeling moieties. A second specific aim is to determine pharmacological activity of APB and glutamate analogues on APB-sensitive neuronal pathways. The experimental system will be hippocampal slices from brains of rats and guinea pigs. The model pathway will be the synaptic termination field of perforant path axons on dentate granule cells. The activity and potency of APB analogues on this system will be measured using electrophysiological techniques. An APB-sensitive pathway measured by stimulating in the hilus of area dentata and recording in area CA3 of the hippocampus will also be investigated. A third specific aim is characterization of APB binding sites associated with biochemical preparations of synaptic plasma membranes. Analogues of APB will be ranked for potency as displacers of glutamate and APB binding in these preparations. Site-specific labeling of APB binding sites with sulfhydryl reagents and APB or glutamate analogues with alkylating and photoaffinity-labeling functions will be attempted. Proteins thus labeled will be isolated and characterized by methods of protein chemistry. These binding sites will also be localized in tissue sections by techniques for light microscopic receptor autoradiography. The proposed studies may have applications which extend beyond these immediate aims. Drugs which interact with specific neural pathways may be useful for experimental physiology and psychology. Ultimately, they may be useful clinically, for example, for control of seizures.